1. Field of the Invention
The present invention relates to an apparatus and method for probing integrated circuits using laser illumination.
2. Description of the Related Art
Probing systems have been used in the art for testing and debugging integrated circuit (IC) designs and layouts. Various laser-based systems for probing IC's are known in the prior art. While some description of the prior art is provided herein, the reader is encouraged to also review U.S. Pat. Nos. 5,208,648, 5,220,403 and 5,940,545, which are incorporated herein by reference in their entirety. Additional related information can be found in Yee, W. M., et al. Laser Voltage Probe (LVP): A Novel Optical Probing Technology for Flip-Chip Packaged Microprocessors, in International Symposium for Testing and Failure Analysis (ISTFA), 2000, p 3-8; Bruce, M. et al. Waveform Acquisition from the Backside of Silicon Using Electro-Optic Probing, in International Symposium for Testing and Failure Analysis (ISTFA), 1999, p 19-25; Kolachina, S. et al. Optical Waveform Probing—Strategies for Non-Flipchip Devices and Other Applications, in International Symposium for Testing and Failure Analysis (ISTFA), 2001, p 51-57; Soref, R. A. and B. R. Bennett, Electrooptical Effects in Silicon. IEEE Journal of Quantum Electronics, 1987. QE-23(1): p. 123-9; Kasapi, S., et al., Laser Beam Backside Probing of CMOS Integrated Circuits. Microelectronics Reliability, 1999. 39: p. 957; Wilsher, K., et al. Integrated Circuit Waveform Probing Using Optical Phase Shift Detection, in International Symposium for Testing and Failure Analysis (ISTFA), 2000, p 479-85; Heinrich, H. K., Picosecond Noninvasive Optical Detection of Internal Electrical Signals in Flip-Chip-Mounted Silicon Integrated Circuits. IBM Journal of Research and Development, 1990. 34(2/3): p. 162-72; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway, Noninvasive sheet charge density probe for integrated silicon devices. Applied Physics Letters, 1986. 48(16): p. 1066-1068; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway, Erratum to Noninvasive sheet charge density probe for integrated silicon devices. Applied Physics Letters, 1986. 48(26): p. 1811.; Heinrich, H. K., et al., Measurement of real-time digital signals in a silicon bipolar junction transistor using a noninvasive optical probe. IEEE Electron Device Letters, 1986. 22(12): p. 650-652; Hemenway, B. R., et al., Optical detection of charge modulation in silicon integrated circuits using a multimode laser-diode probe. IEEE Electron Device Letters, 1987. 8(8): p. 344-346; A. Black, C. Courville, G Schultheis, H. Heinrich, Optical Sampling of GHz Charge Density Modulation in SIlicon Bipolar Junction Transistors Electronics Letters, 1987, Vol. 23, No. 15, p. 783-784, which are incorporated herein by reference in their entirety.
As is known, during debug and testing of an IC, a commercially available Automated Testing Equipment, also known as an Automated Testing and Evaluation (ATE) tester, is used to generate test patterns (also referred to as test vectors) to be applied to the IC device under test (DUT). When a laser-based system is used for probing, the DUT is illuminated by the laser and the light reflected from the DUT is collected by the probing system. As the laser beam strikes the DUT, the laser beam is modulated by the response of various elements of the DUT to the test vectors. This has been ascribed to the electrical modulation of the free carrier density, and the resultant perturbation of the index of refraction, of the material. Accordingly, analysis of the reflected light provides information about the operation of various devices in the DUT.
FIG. 1 is a general schematic depicting major components of a laser-based voltage probe system architecture, 100, according to the prior art. In FIG. 1, dashed arrows represent optical path, while solid arrows represent electronic signal path. The optical paths represented by curved lines are generally made using fiber optic cables. Probe system 100 comprises a dual laser source, DLS 110, an optical bench 112, and data acquisition and analysis apparatus 114. The optical bench 112 includes provisions for mounting the DUT 160. A conventional ATE tester 140 provides stimulus signals and receives response signals 142 to/from the DUT 160 and provides trigger and clock signals, 144, to the time-base board 155. The time-base board 155 synchronizes the signal acquisition with the DUT stimulus and the laser pulses. Workstation 170 controls as well as receives, processes, and displays data from the signal acquisition board 150, time-base board 155, and the optical bench 112.
The various elements of probe system 100 will now be described in more detail. Since temporal resolution is of high importance in testing DUT's, the embodiment of FIG. 1 utilizes prior art pulsed lasers, wherein the laser pulse width determines the temporal resolution of the system. Dual laser source 110 consists of two lasers: a pulsed mode-locked laser, MLL 104, source that is used to generate 10-35 ps wide pulses, and a continuous-wave laser source, CWL 106, that can be externally gated to generate approximately 1 μs wide pulses. The MLL 104 source runs at a fixed frequency, typically 100 MHz, and must be synchronized with the stimulus 142 provided to the DUT 160, via a phase-locked loop (PLL) on the time-base board 155, and the trigger and clock signals 144 provided by the ATE tester. The output of the DLS 110 is transmitted to the optical bench 112 using fiber optics cable 115. The light beam is then manipulated by beam optics 125, which directs the light beam to illuminate selected parts of the DUT 160. The beam optics 125 consists of a Laser Scanning Microscope (LSM 130) and beam manipulation optics (BMO 135). The specific elements that are conventional to such an optics setup, such as objective lens, etc., are not shown. Generally, BMO 135 consists of optical elements necessary to manipulate the beam to the required shape, focus, polarization, etc., while the LSM 130 consists of elements necessary for scanning the beam over a specified area of the DUT. In addition to scanning the beam, the LSM 130 has vector-pointing mode to direct the laser beams to anywhere within the field-of-view of the LSM and Objective Lens. The X-Y-Z stage 120 moves the beam optics 125 relative to the stationary DUT 160. Using the stage 120 and the vector-pointing mode of the LSM 130, any point of interest on the DUT 160 may be illuminated and probed.
For probing the DUT 160, the ATE 140 sends stimulus signals 142 to the DUT, in synchronization with the trigger and clock signals provided to the phase-locked loop on the time-base board 155. The phase-lock loop controls the MLL 104 to synchronize its output pulses to the stimulus signals 142 to the DUT. MLL 104 emits laser pulses that illuminate a particular device of interest on the DUT that is being stimulated. The reflected light from the DUT is collected by the beam optics 125, and is transmitted to photodetector 138 via fiber optic cable 134. The reflected beam changes character depending on the reaction of the device to the stimulus signal. To monitor incident laser power, for purposes of compensating for laser power fluctuations, for example, optical bench 112 provides means to divert a portion of MLL 104 incident pulse to photodetector 136 via fiber optic cable 13. The output signal of the photodetectors 132, 134 is sent to signal acquisition board 150, which, in turn, sends the signal to the controller 170. By manipulation of the phase lock loop on the time-base board 155, controller 170 controls the precise time position of MLL 104 pulses with respect to DUT 160 stimulus signals 142. By changing this time position and monitoring the photodetectors signals, the controller 170 can analyze the temporal response of the DUT to the stimulus signals 142. The temporal resolution of the analysis is dependent upon the width of the MLL 104 pulse.
A major difficulty encountered by all laser-base probe systems is deciphering the weak modulation in the reflected signal, which is caused by the response of the DUT to the stimulus. Another difficulty is noise introduced into the signal by the DUT's vibrations. Various beam manipulation optic, 135, designs have been used in the art in an attempt to solve these difficulties. FIG. 2 is a diagram illustrating standard amplitude detection mode used in the prior art. In FIG. 2, a laser probe is used to probe specific device 210, such as a transistor's gate or drain, in a DUT. A beam splitter 220 is used to separate the reflected beam from the incident laser beam. Amplitude modulation due to DUT interaction with the laser beam can be detected directly using a photodetector. However, DUT vibrations cause amplitude variations that are much stronger than the variation from the DUT activity of interest. This necessitates noise rejection schemes to make such an arrangement practical. One scheme implemented in the prior art is dual-laser noise rejection. In dual-laser noise rejection, the pulses from CWL 106 are used to measure the DUT 160 vibrations. The CWL measurements are then used to normalize the MLL measurements of the DUT activity.
The DUT interaction with the laser beam may cause changes mostly in the phase of the reflected laser beam, not its amplitude. Consequently, the signal strength may be too weak for pure amplitude detection. Various phase detection schemes have been developed for the beam manipulation optics 135. FIG. 3 is a diagram illustrating a phase detection scheme using a Michelson Interferometer arrangement to convert phase to amplitude. This scheme is also referred to as Phase-Interferometric Detection, or PID, mode. To detect phase modulations, a portion of the incident beam from the laser source is directed into a reference arm consisting of a lens 340 and a mirror 330, using beam splitter 320. The remaining portion of the incident beam is directed to a specific device of interest on the DUT, and upon reflection it is modulated according to the DUT's response to a stimulus signal. The light beam, 355, reflected by the DUT, and the light beam, 335, reflected by the reference arm mirror 330, are spatially recombined into a single beam 365 so that they can interfere (light beams 355 and 335 are shown spatially separated in FIG. 3 for illustrative purposes). The interference effect converts relative phase differences between the reflected beam 355 and the reference arm beam 335 into amplitude differences in resultant beam 365, which can then be detected by a photodetector.
While this arrangement helps detect phase variations caused by the DUT, using this optical arrangement exposes the system to additional noise source from phase variations caused by DUT vibrations. The DUT vibrations still modulate reflected DUT beam amplitude, but now also modulate the DUT beam phase, which generates larger resultant beam 365 amplitude modulations. To compensate for this phase noise, a modified dual-laser noise rejection scheme is used. In this modified scheme, the CWL resultant signal is used in a feedback loop to control reference arm mirror 330 position. By striving to maintain constant CWL resultant signal, the feedback loop drives reference arm mirror 330 to track DUT vibrations in order to maintain a constant quiescent phase offset value between DUT and reference arms. Fine control of mirror 330 position for the feedback loop is provided via a piezo electric transducer element (not shown). Additional adjustments (not shown) that are required in order to get best performance include reference arm power control and coarse reference arm mirror position control. Reference arm power control allows the reflected power from the DUT and reference arm mirror to be matched. Coarse reference arm mirror position control allows DUT and reference arm optical path lengths to be nominally equalized, a necessity for the operation of the modified dual-laser noise reduction scheme due to wavelength differences between MLL and CWL used in the prior art. Means to align optics to ensure overlap of reflected DUT and reference arm beams are also necessary for best performance.
FIG. 4 is a diagram illustrating another scheme, generally known as (spatial) differential probing (SDP) for phase detection. A Wollaston prism, contained within beam manipulation optics 430, is used to generate the two spatially separated beams, 422, 424. The two beams, 422 and 424, have orthogonal, linear polarization states (denoted by the dots and arrows of beams 424 and 422, respectively). One beam, e.g., 422, is directed to the DUT active device of interest; while the other beam, say, 424, can be directed to either an inactive device or region, or to an active device with complementary modulation. The advantage of the latter option is that the measured signal modulation is increased because the relative phase modulation between the two beams is doubled. In the particular example of FIG. 4, the two beams are directed to the drains of the p- and n- FETs of an inverter, which generates complementary modulations of the beam. Beam manipulation optics 430 spatially recombines the two reflected beams, 435, 455, and converts them to the same polarization state so that they may interfere with each other to generate amplitude modulated resultant beam 465. Beam manipulation optics 430 also provides means to introduce a phase offset between the two beams so that the interference condition can be optimized for maximum signal sensitivity. Using this scheme the phase noise is reduced relative to the scheme illustrated in FIG. 3, because each beam is directed at the DUT, so that the DUT vibrations will tend to modulate the phase of both beams similarly.
As can be understood, various IC's have different layouts, and different devices within an IC's have different dimensions and surroundings. Therefore, using this embodiment, for each device to be probed the user needs to decide where to place each beam within the chip. Moreover, since the beam needs to be placed at various locations in the chip, the system needs to be designed so that the beam separation is adjustable, which complicates the optics design. Additionally, the intensity ratio of the beams must be variable since the reflectivity of the regions where they are placed can differ. Power matching between the two beams is required for best results.
Experience with devices as depicted in FIG. 4 has shown that DUT vibrations can still generate amplitude fluctuations if the two laser beams are not incident on identical structures. Due to variations in the layout and dimensions of the DUT, it is difficult to find identical structures for probing within the range of adjustment of the two laser beams. Differences in structure cause differences in reflection as the DUT is vibrated. To reduce the effects of these vibrations, the dual-laser noise reduction scheme can be used. However, the different polarization states of the two beams also makes them respond differently to time-varying birefringence in the DUT or in the optics of the system. Time varying birefringence in the DUT can be caused by stress variations due to mechanical vibrations, for example.
FIG. 5 is a diagram illustrating time differential probing (TDP) scheme for phase detection. Two pulsed beams, 522 and 524, are generated by splitting a single pulse beam in two and time shifting one relative to the other by a small amount (approx. 10-100 ps) before DUT interaction. Time shifting can be achieved by passing one beam through an optical delay line (such as a thick piece of glass) contained within beam manipulation optics, 526. The two beams are spatially recombined after splitting so that they traverse a common path to the DUT. After DUT reflection, the opposite beam is passed through the optical delay line to remove the relative delay between the two beams. The two beams are then allowed to interfere, 530. Since the two beams traverse a common path, DUT vibrations largely modulate both identically, making this scheme inherently vibration insensitive. However, the optics required to generate the delay, and then reverse its effect for the reflected beams, are relatively complicated. Additionally, the beams are not completely identical. They have orthogonal polarization states so DUT interactions are not truly identical (birefringence effects can cause non-common mode variations of the beams). In this scheme, the resulting ‘waveforms’ are derivatives of the signal at the probed device in the DUT and typically consist of positive and negative going peaks in the case of probed logic signals, for example. When both beams are position in time on logic highs or lows, the resultant beam intensity is the same since both beams are phase shifted identically in each case. Only when the time shifted beams span a logic transition (one beam on a logic high, the other beam on a logic low) or part of a logic transition, does a phase difference result. This arrangement requires high temporal resolution of the sampling to ensure that each logic transition of interest is spanned by the time separated beams. This can limit the maximum time span of a sampling window that can be used, or may require additional acquisition time to fill the desired sampling window with enough density of sampling points. In addition, the temporal resolution (or measurement bandwidth) of this scheme is limited by the pulse separation used instead of by the width of the laser pulses. Since larger pulse separations typically give better signal strength, this scheme suffers from a measurement bandwidth versus signal strength trade-off.
Accordingly, there is a need in the art for a system that will allow improved laser probing of a DUT, while simplifying operation and minimizing the system's complexity and cost.